Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as energy sources. Among these secondary batteries, lithium secondary batteries having high energy density and voltage, long lifespan and low self-discharge are commercially available and widely used.
In addition, increased interest in environmental issues has brought about a great deal of research associated with electric vehicles (EVs) and hybrid electric vehicles (HEVs) as substitutes for vehicles using fossil fuels such as gasoline vehicles and diesel vehicles which are main factors of air pollution. These electric vehicles generally use nickel metal hydride (Ni-MH) secondary batteries as power sources of electric vehicles (EVs), hybrid electric vehicles (HEVs) and the like. However, a great deal of study associated with use of lithium secondary batteries with high energy density and discharge voltage is currently underway and some are commercially available.
In particular, lithium secondary batteries used for electric vehicles should have high energy density, exhibit great power within a short time and be used for 10 years or longer under harsh conditions in which charge and discharge based on high current is repeated within a short time, thus requiring considerably superior stability and long lifespan, as compared to conventional small lithium secondary batteries.
As the cathode active material of lithium ion secondary batteries used for small batteries, lithium-containing cobalt oxide such as LiCoO2 having a layered structure is used. In addition, use of lithium-containing manganese oxides such as LiMnO2 having a layered crystal structure and LiMn2O4 having a spinel crystal structure, and lithium-containing nickel oxide (LiNiO2) has been considered.
Among these cathode active materials, LiCoO2 is the most generally used owing to lifespan properties and high charge and discharge efficiency, but has disadvantages of low structural stability and cost due to resource limitations of cobalt used as a raw material and thus limitation of price competitiveness.
Lithium manganese oxides such as LiMnO2 and LiMn2O4 have advantages of superior thermal stability and low cost, but having disadvantages of small capacity and low high-temperature properties.
In addition, LiNiO2 cathode active materials exhibit better properties of superior charge capacity, but are considerably difficult to synthesize due to the problem of cation mixing between Li and transition metal, thus having great problems associated with rate characteristics.